Disposable electrode

ABSTRACT

The disposable medical electrode includes a disk formed of a semi-rigid plastic material having a recessed embossment located centrally thereof. A metallic electrical contact is mounted on the disk centrally of the embossment and has portions thereof located on both sides of the disk. An absorbent pad positioned in the recessed embossment has absorbed therein an electrically conductive gel for forming an electrical contact between the patient on whom the electrode is positioned and the electrical contact thereby to permit electrical signals from the patient to be detected with the electrode by appropriate electrical equipment connected thereto. The disk has an adhesive applied thereto for firmly securing the electrode to the patient&#39;&#39;s skin.

nited States Patent 1191 Buchalter Aug. 26, 1975 1 DISPOSABLE ELECTRODE 22 Filed: Oct. 26, 1973 21 Appl. No.: 409,884

[52] 11.5. CI. l28/2.06 E; 128/2.l E; 128/417; 128/418; l28/DIG. 4 [51] Int. Cl. A6lB 5/04 [58] Field of Search 128/2.06 E, 2.1 E, DIG. 4, 128/417, 418, 404, 172.1, 410, 411

[56] References Cited UNITED STATES PATENTS 2,632,447 3/1953 Dobcs 128/404 3,187,745 6/1965 Baum et al. v 128/2.06 E

3,279,468 10/1966 Lc Vine v 128/410 3,518,984 7/1970 Mason 128/2.06 E 3,701,346 10/1972 Patrick ct a1 l28/2.06 E 3,788,317 1/1974 McCormick 128/2.06 E

FOREIGN PATENTS OR APPLICATIONS 675,494 12/1963 Canada 128/417 Primary ExaminerRichard A. Gaudet Assistant ExaminerLee S. Cohen Attorney, Agent, or FirmLewis H. Eslinger; Alvin Sinderbrand [5 7 ABSTRACT The disposable medical electrode includes a disk formed of a semi-rigid plastic material having a recessed embossment located centrally thereof. A metallic electrical contact is mounted on the disk centrally of the embossment and has portions thereof located on both sides of the disk. An absorbent pad positioned in the recessed embossment has absorbed therein an electrically conductive gel for forming an electrical contact between the patient on whom the electrode is positioned and the electrical contact thereby to permit electrical signals from the patient to be detected with the electrode by appropriate electrical equipment connected thereto. The disk has an adhesive applied thereto for firmly securing the electrode to the patients skin.

9 Claims, 5 Drawing Figures 1 DISPOSABLE ELECTRODE The present invention relates to medical electrodes, and more particularly to pre-wet or pre-gelled disposable medical electrodes designed for use in short or long term applications.

The use of disposable metal electrodes for monitoring a patient, taking electrocardiograms and other medical diagnostic information has recently developed in connection with expanded technology which permits such diagnostic procedures and monitoring to take place at remote locations, in the home, ambulance, emergency room of a hospital, an intensive care unit, etc. Pre-wet or pre-gelled disposable electrodes have also been found to be more convenient to transport to remote locations than conventional electrodes and can be discarded immediately after use so that no special storage facilities are required in the physicians bag or ambulance. Such pre-wet electrodes, whether used in the hospital itself or at a remote location, eliminate the need to apply an electrode gel, paste or solution to the electrode or patient prior to placing the electrodes on the patient. Moreover, by the use of disposable electrodes, the diagnostic unit can be immediately reused on successive patients without the electrodes being sterilized or otherwise cleaned for use on the new patient, since disposable electrodes can simply be discarded and new ones used in place thereof.

Previously proposed disposable electrodes have been supplied in many forms. Typically, the electrodes are formed as a relatively large diameter disk of foamed plastic material or adhesive tape having a hard rigid plastic receptacle mounted at the center thereof for containing an absorbent electrode pad. This pad normally is secured to the plastic receptacle by an adhesive or the like and contains an electrically conductive gel or fluid absorbed therein, and it is positioned in contact with a metallic electrode which extends through the disk. The connector is then used to electrically connect the electrode to the diagnostic or monitoring unit, for example to an ECG, an ECG monitor or the like. The absorbent pad or sponge in the hard rigid disk mounted on the electrode provides the electrical connection between the skin of the patient and the electrical connector.

Typically, disposable electrodes of this type, i.e. formed of a sponge or foamed plastic disk or as a disk of adhesive tape, are supplied on a sheet of paper, with the side of the electrode containing the sponge contact being adhesively secured to a carrier sheet and with the sponge contact exposed through an aperture in the carrier sheet. A rigid plastic cap member is usually secured to the disk of the electrode in order to protect the gel in the sponge against deterioration and to prevent evaporation of the gel absorbed therein until the electrode is used. Accordingly, it will be appreciated that such previously proposed disposable electrodes are relatively expensive to manufacture, requiring numerous components and are relatively difficult and expensive to assemble, particularly because of the provision of a special disk to hold the absorbent pad or sponge and the consequential need for a cover cap to protect the sponge and gel.

Another type of previously proposed disposable electrode simply consists of the foamed plastic disk, with a metallic connector extending therethrough. The metallic connector is then placed directly in contact with the patients skin when the electrode is used. Not only is the metallic electrode in contact with the skin in this manner irritating and uncomfortable for the patient, but also the quality of signals detected, because of the typically poor contact and conductivity between the metal contact of the electrodes and the skin, is sometimes unacceptable. This type of electrode sometimes has a porous paper disk pre-wetted with a conductive gel covering the metallic electrode. However, the paper disk has insufficient thickness to absorb or hold enough electrolytic gel so that uniform or sufficient contact with both the skin of the patient and the surface of the electrode is not provided.

In yet another form of previously proposed disposable electrodes, the electrode is formed simply as a paper disk having a metallic connector extending therethrough. Electrodes of this type suffer from the same disadvantages as the above discussed electrodes and, moreover, thepaper disk is readily and easily torn or damaged during handling, particularly when it is pulled from the adhesive carrier sheet on which the electrode is supplied to the physician. Still further, all of the previously proposed disposable electrodes are formed with a relatively large diameter so that it is not possible to simultaneously place six chest electrodes across a patients chest as is required in recently devel oped three channel electrocardiogram equipment or with conventional l0 lead equipment where all of the leads are simultaneously connected to the patient but sequentially read.

Accordingly, it is apparent that although a variety of previously proposed disposable electrodes have been used in the past, such electrodes have not been entirely satisfactory in use even though a substantial demand for such electrodes is evident in the medical field.

Accordingly, it is an object of the present invention to provide a disposable electrode which has substantial strength and yet is inexpensive to manufacture.

Another object of the present invention is to provide a disposable medical electrode which is relatively simple in construction and durable in use.

Another object of the present invention is to provide a relatively inexpensive yet durable disposable medical electrode which is highly suitable for use in monitoring a patients condition.

Yet another object of the invention is to provide a disposable electrode which is of relatively small dimensions to permit use with conventional ten lead electrocardiogram equipment and/or with the three channel EKG equipment.

In accordance with one aspect of the present invention, the disposable metal electrode provided thereby includes a circular support disk defining the electrode which is formed of a semi-rigid hard plastic material. The disk has an integrally formed embossed recess at its central portion, which recess may be generally cylindrical or square in shape. The recess opens towards one side of the disk so that the opposite side of the disk has a raised central portion formed therein. The disk has an aperture formed therein at the center of the recess and a two-piece electrical contact member is secured to the raised central portion of the disk and includes a first contact piece located within the recess on one side of the disk. This first contact piece includes an extension pin which extends through the aperture in the electrode disk into engagement with a second contact piece located on the opposite side of the disk. This second electrode includes a female extension which receives and mates with the extension member of the first contact piece in order to form an electrical connection therebetween.

An absorbent pad of a sponge or sponge-like material is positioned in the embossed recess of the disk in contact with the first electrical contact piece and is preferably held there snugly by a friction fit. The sponge has absorbed therein an electrically conductive gel so that when the electrode is secured to a patient, with the sponge in contact with the patients skin, an electrical connection is formed between the skin and the second contact piece thereby to permit electrical signals from the patient to be detected. Accordingly, the disposable electrode of the present invention is formed from a minimum of inexpensively manufactured parts.

The above, and other objects, features and advantages of the present invention will be apparent in the following detailed description of an illustrative embodiment thereof, which is to be read in connection with the accompanying drawings, wherein:

FIG. 1 is a partial plan view of a patient having a plurality of disposable electrodes constructed in accordance with the present invention adhered to his chest for the purpose of patient monitoring;

FIG. 2 is a perspective view of a disposable electrode constructed in accordance with one embodiment of the present invention and an electrical connector which connects the electrode to the diagnostic device;

FIG. 3 is a perspective view of a disposable electrode constructed in accordance with another embodiment of the invention;

FIG. 4 is a sectional view of a disposable medical electrode of the present invention, taken along line 44 of FIG. 2 prior to application of the electrode to the patient; and

FIG. 5 is a sectional view similar to FIG. 4 showing the electrode as mounted on a carrier sheet.

Referring to the drawing in detail, and initially to FIG. 1 thereof, it will be seen that disposable electrodes 10, constructed in accordance with the present invention, are adapted to be placed on the chest wall of a patient 12, in order to detect or monitor electrical signals formed in the body or to apply electrical impulses to the body, such as in muscle stimulation or electrical nerve block techniques. Typically, the electrodes of the present invention are used to monitor a patients condition in a hospital room or in intensive care, or to monitor stress in a patient or subject during exercise and the like or to take an electrocardiogram from a patient in the conventional manner or to provide electrical impulses to the body. Thus, the electrodes are connected through a plurality of electrical conductors or leads 14 to a plug cable 16 of conventional construction from which the signals can be supplied to the diagnostic unit or device to a telephone line or the like for transmission to the hospital or main diagnostic apparatus, computer, etc. As a result, the technician, nurse or physician is able to monitor the patients condition or record the patients electrocardiogram, even though the display device or electrocardiogram machine itself is not present with the patient.

As seen in FIG. 2, the electrode of the present invention includes a main disk 18 which is formed of a semirigid relatively hard plastic sheet material. This material may, for example, be formed of polyvinylchloride, polyethylene, or other conveniently available thermoformable plastic material which have some flexibility, so as to permit the disk to conform to the contours of the patients body on which it is placed, and yet are also relatively hard so that they are not torn or damaged when used and handled. Such plastic materials preferably form a vapor barrier to prevent drying or evaporation of the gel in the electrode during storage or use. Drying of such gel is a principal problem with previously proposed electrodes requiring special caps or seals which are avoided by the present invention.

Disk 18 preferably has a diameter of approximately 1 /2 inches and has a central embossment 20 formed therein which, as seen in FIG. 3, opens towards one side 22 of disk 18. The use of a small diameter for the disk 18 has several important advantages including that it permits a hard plastic material to be used so as to form a vapor barrier with the small size being such as to be able to accommodate body curvature at the point of application of the electrode by slight flexing of the disk. In addition, the small size of the electrodes permit a plurality of the electrodes (e.g. five or six) to be simultaneously placed across the chest wall of the patient in order to permit a conventional ten lead ECG to be taken or to allow use of recently developed three channel ECG machines. Moreover, the electrode is light in weight and can flex with body movements so that it is relatively comfortable on the skin and to the touch.

The embossments 20 can be formed in a generally cylindrical configuration as seen in FIG. 3, or in a square configuration as shown in FIG. 4. The square embodiment is advantageous since it provides a greater surface area for the recess and therefore the absorbent sponge therein for a given diameter or maximum side dimension. In either case, the embossment 20 is formed in any convenient manner, such as for example, by thermoforming the sheet material of which the disks 18 are formed. By this construction, a central recess 24 is provided having an annular flange 26 extending thereabout. The recess 24 is located substantially coaxially with the disk 18, and has an aperture 28 formed therein at its center.

Aperture 28, which may be preformed during the thermoforming operation or die cut after that operation, permits the mounting of a two-piece metal electrical connector member 30 on the disk 18. Connector 30 includes a first connector piece 32 which consists of an annular disk 34 having a male extension 36 extending therefrom in a generally friction tight relation through the aperture 28. A second connector piece 38 is located on the opposite side of the disk 18 from connector piece 34 and has a female extension 40 which receives the extension 36 of connector piece 32 in mating engagement. In this manner the two contact pieces are in a snap fitting relationship, so that they essentially clamp themselves onto the disk 18 at the center thereof in a rigid mating relationship with the disk 18 sandwiched therebetween. Moreover, the contact pieces, as seen in FIG. 4, form a seal with each other and with disk 18 to prevent or limit evaporation of the gel in the electrodes recess 24 through opening 28 or through the stems of the contact pieces themselves.

The recess or cavity 24 provides a retention pocket for an absorbent pad 46 which may be formed of a natural or synthetic sponge material. The sponge substantially fills the recess 24 as seen in FIG. 3, and may be cut in a slightly larger diameter than that ofrecess 24 so as to be frictionally held in the recess. In that case, the sponge will bulge slightly, as shown in FIG. 3, out of recess 24. However, when placed on the body the bulge will insure proper contact of the sponge with the skin. Moreover, the securement of the disk to the patients skin by the adhesive on the disk, as described hereinafter, will urge the sponge into recess 24 so that it also is in contact with connector 30.

In addition, sponge 46 has absorbed therein an electrically conductive gel material which is commercially available and well known in the art, such as for example, hydroxyethyl cellulose or any of the cellulose productswith /2 to 5% sodium potassium or calicum chloride or combinations thereof, plus a suitable preservative and a umectant. In another embodiment a chloride-free electrically conductive gel may also be used in sponge 46, such as for example, Spectra 360 Electrode gel sold by Parker Laboratories.

Although previously proposed electrodes use silver or silver plated contact members, or nickel contact members with a non-chloride gel to prevent corrosion of the nickel, applicant has found that a nickel or brassnickel plated contact can be used in the electrode with a chloride or low chloride type gel while avoiding corrosion or rust in the contact.

By the provision of the absorbing gel in sponge 46 an electrical connection is made between the outer surface 48 of the sponge (which is placed in contact with the body) and the electrical connector member 30 so that electrical signals from the patient can be detected and supplied to the electrocardiogram by proper connection with the connector 30.

For example, as illustrated in FIG. 2, a commercially available wire lead 50 is provided which has an annular connector member 52 having a central aperture 54 therein which is adapted to receive the extension 40 of electrode 10. In the embodiment of the wire electrode 50 illustrated in the drawing, a pair of wire contacts 56 extend along opposite sides of the opening 54 so as to snap over the enlargement 44 at the top of the electrode 10, thereby to insure proper electrical contact between the contact extension 40 and connector 52. The opposite end of the wire lead 50 can be provided with a male plug 58, for insertion in the plug cable system 16 so that the signals detected by the various leads can be sent over a common line 60 to the electrocardiogram at the hospital through a telephone line or the like.

The electrodes of the present invention are typically supplied with a plurality of the electrodes adhesively secured to a carrier or support sheet 62 by a pressure sensitive adhesive or the like which is medically inactive and which will not effect the patient. The carrier sheet preferably is formed from a-polyethylene silicone coated release paper such as is available from the H. P. Smith Division of Philips Petroleum. Such paper sheet is either impregnated with polyethylene or laminated to a polyethylene sheet and then coated with a silicone release coating on one side thereof. This release coating, or the under side of the electrodes 10 surrounding the recesses therein, then have an adhesive layer 63 applied thereto in any convenient manner so as to secure the electrodes to the carrier sheet, as seen in FIG. 5. Carrier sheet 62 thus closes the opened end of recess 24 and thus forms a vapor barrier so as to pro tect the gel in sponge 46 from deterioration or evaporation in the air.

When it is desired to use the electrode 10, the electrode or electrodes are simply peeled from the carrier sheet 62 and applied to the patients body. It is noted that the same adhesive which secures the electrode to the carrier sheet serves to firmly adhere the electrode in position on the patient. The adhesive layer 63 which is used between the electrode and the carrier sheet can be any of the commercially available pressure sensitive adhesives which are FDA approved for application to the body.

It is noted that the carrier 62 can also be formed of a plastic sheet material, or alternatively, can be formed of paper, treated, in either case, with a silicon layer on the side to which the electrode is adhered to permit removal of the electrode from the sheet. However, it is preferred that a plastic or plastic impregnated sheet material be utilized because it has additional strength over paper and because it forms a vapor barrier which will limit or even prevent evaporation of the gel electrode during storage.

Accordingly, it is seen that a relatively simply constructed electrode, having a minimum of elements, is provided which will be relatively durable in use. By forming the electrode of a semi-rigid, but hard plastic material, the electrode will not be damaged or torn during handling or removal from the carrier sheet. In addition, since the electrode is a semi-rigid and hard plastic material, it can be embossed to form the integral recess 24 therein so as to provide a carrier and support pocket for the electrode sponge 46. This material thus forms an integral vapor barrier in the electrode to limit or prevent evaporation of the gel through the electrode material. This is in contradistinction to previously proposed disposable electrodes wherein a separate hard plastic ring or cavity member is secured to a sponge or foam plastic material and closed by a hard plastic cap in order to support the electrode sponge in the electrode during storage and to limit evaporation of the gel. Accordingly, the present invention provides an inexpensive method of manufacturing electrodes using gel carrier sponges by eliminating the rings and caps required in the prior art while at the same time retaining the use and advantages of the sponge carrier as compared to the prior art devices in which no carrier or only a flat paper disk is used for the gel.

Although an illustrative embodiment of the present invention has been described herein with reference to the accompanying drawing, it is to be understood that the invention is not limited to that precise embodiment and that various changes and modifications may be effected therein by one skilled in the art without departing from the scope or spirit of this invention.

What is claimed is:

l. A disposable medical electrode comprising a disk of hard semi-rigid material having a recessed embossment formed centrally thereof and opening towards the side of the disk to be secured to a patient and a flat peripheral flange completely surrounding the embossment for securing the disk to the patient, a metallic electrical contact mounted on said disk centrally of said recessed embossment and having portions thereof located on opposite sides of said embossment, and an absorbent pad positioned in and substantially filling said recessed embossment, said pad being in electrical contact with said metallic electrical contact and having absorbed therein an electrically conductive gel; said flat peripheral flange having a flat outer surface surin the rounding said recess on said one side of the disk, said flat surface having an adhesive coating thereon; and a carrier sheet formed of a plastic sheet material having a release coating thereon engaged and releasably secured to the entire flat surface of said peripheral flange on said one side of the disk, covering the recess and pad therein and sealing said recess about the entire periphery thereof on said one side of the disk whereby the pad and gel in the recess are protected and sealed therein with a vapor barrier on all sides thereof formed by said disk and carrier sheet to prevent evaporation of gel in the recess prior to use of the disk, said adhesive coating on the flat surface of said peripheral flange serving to adhere the disk to the patient when in use.

2. The electrode as defined in claim 1 wherein said absorbent pad is a sponge.

3. The electrode as defined in claim 2 wherein said disk is circular in plan and said recessed embossment is generally cylindrical in shape having an open end opening towards said one side of said disk.

4. The electrode as defined in claim 2 wherein said electrical contact member comprises a two section contact having a first section located in said recessed embossment on said one side of said disk including an extension pin extending through said disk to the opposite side thereof and a second section located on the opposite side of said disk including a female extension receiving said extension pin in mating engagement to form an electrical connection therebetween, said absorbent pad and gel therein being positioned in contact with said first section of said contact member to provide electrical communication therebetween.

5. The electrode as defined in claim 2 wherein said disk is circular in plan and said recessed embossment is generally square in plan, having an open end opening towards said one side of said disk.

6. The electrode as defined in claim 1 wheein said re lease coating is a silicone release coating.

7. A disposable medical electrode comprising, in combination, a circular support disk formed of a semirigid hard plastic material, said disk having an embossed recess formed at a central portion of the disk opening towards one side of said disk whereby the opposite side of said disk has a raised central portion formed therein, said disk having an aperture therein at the center of said recess and a flat peripheral flange completely surrounding the embossment for securing the disk to the patient; a two-piece electrical contact member secured to said raised central portion of the disk including a first contact piece located in said recess on said one side of the disk including an extension pin extending through said apertures and a second contact piece located on the opposite side of said disk including a female extension member receiving and mating with said extension member of said first contact piece to form an electrical connection therebetween; and an absorbent sponge positioned in said recess in contact with said first contact piece, said sponge having absorbed therein an electrically conductive gel whereby, when said one side of the electrode is secured to a patient with said sponge in contact with the patients skin, an electrical connection is formed between the skin and said second contact piece thereby to permit electrical signals from the patient to be detected; said flat peripheral flange having a flat outer surface surrounding said recess on said one side of the disk, said flat surface having an adhesive coating thereon; a carrier sheet formed of a plastic sheet material having a silicone release coating thereon engaged and releasably secured to said adhesive coating on the entire flat surface of said peripheral flange, said carrier sheet extending across the entire recess and covering said recess and sponge therein thereby sealing said recess about the entire periphery thereof on said one side of the disk whereby the pad and gel in the recess are protected and sealed therein with a vapor barrier formed on all sides thereof by said disk and carrier sheet to prevent evaporation of gel in the recess prior to use of the disk, said adhesive coating on the flat surface of said peripheral flange serving to adhere the disk to the patient when the disk is in use.

8. The electrode as defined in claim 7 wherein said embossed recess is generally circular in plan.

9. The electrode as defined in claim 7 wherein said embossed recess is square in plan. 

1. A disposable medical electrode comprising a disk of hard semi-rigid material having a recessed embossment formed centrally thereof and opening towards the side of the disk to be secured to a patient and a flat peripheral flange completely surrounding the embossment for securing the disk to the patient, a metallic electrical contact mounted on said disk centrally of said recessed embossment and having portions thereof located on opposite sides of said embossment, and an absorbent pad positioned in and substantially filling said recessed embossment, said pad being in electrical contact with said metallic electrical contact and having absorbed therein an electrically conductive gel; said flat peripheral flange having a flat outer surface surrounding said recess on said one side of the disk, said flat surface having an adhesive coating thereon; and a carrier sheet formed of a plastic sheet material having a release coating thereon engaged and releasably secured to the entire flat surface of said peripheral flange on said one side of the disk, covering the recess and pad therein and sealing said recess about the entire periphery thereof on said one side of the disk whereby the pad and gel in the recess are protected and sealed therein with a vapor barrier on all sides thereof formed by said disk and carrier sheet to prevent evaporation of gel in the recess prior to use of the disk, said adhesive coating on the flat surface of said peripheral flange serving to adhere the disk to the patient when in use.
 2. The electrode as defined in claim 1 wherein said absorbent pad is a sponge.
 3. The electrode as defined in claim 2 wherein said disk is circular in plan and said recessed embossment is generally cylindrical in shape having an open end opening towards said one side of said disk.
 4. The electrode as defined in claim 2 wherein said electrical contact member comprises a two section contact having a first section located in said recessed embossment on said one side of said disk including an extension pin extending through said disk to the opposite side thereof and a second section located on the opposite side of said disk including a female extension receiving said extension pin in mating engagement to form an electrical connection therebetween, said absorbent pad and gel tHerein being positioned in contact with said first section of said contact member to provide electrical communication therebetween.
 5. The electrode as defined in claim 2 wherein said disk is circular in plan and said recessed embossment is generally square in plan, having an open end opening towards said one side of said disk.
 6. The electrode as defined in claim 1 wheein said release coating is a silicone release coating.
 7. A disposable medical electrode comprising, in combination, a circular support disk formed of a semi-rigid hard plastic material, said disk having an embossed recess formed at a central portion of the disk opening towards one side of said disk whereby the opposite side of said disk has a raised central portion formed therein, said disk having an aperture therein at the center of said recess and a flat peripheral flange completely surrounding the embossment for securing the disk to the patient; a two-piece electrical contact member secured to said raised central portion of the disk including a first contact piece located in said recess on said one side of the disk including an extension pin extending through said apertures and a second contact piece located on the opposite side of said disk including a female extension member receiving and mating with said extension member of said first contact piece to form an electrical connection therebetween; and an absorbent sponge positioned in said recess in contact with said first contact piece, said sponge having absorbed therein an electrically conductive gel whereby, when said one side of the electrode is secured to a patient with said sponge in contact with the patient''s skin, an electrical connection is formed between the skin and said second contact piece thereby to permit electrical signals from the patient to be detected; said flat peripheral flange having a flat outer surface surrounding said recess on said one side of the disk, said flat surface having an adhesive coating thereon; a carrier sheet formed of a plastic sheet material having a silicone release coating thereon engaged and releasably secured to said adhesive coating on the entire flat surface of said peripheral flange, said carrier sheet extending across the entire recess and covering said recess and sponge therein thereby sealing said recess about the entire periphery thereof on said one side of the disk whereby the pad and gel in the recess are protected and sealed therein with a vapor barrier formed on all sides thereof by said disk and carrier sheet to prevent evaporation of gel in the recess prior to use of the disk, said adhesive coating on the flat surface of said peripheral flange serving to adhere the disk to the patient when the disk is in use.
 8. The electrode as defined in claim 7 wherein said embossed recess is generally circular in plan.
 9. The electrode as defined in claim 7 wherein said embossed recess is square in plan. 